Magnetic and dielectric properties of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>R</mml:mi><mml:msub><mml:mi mathvariant="normal">Fe</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>,<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>R</mml:mi><mml:mi mathvariant="normal">Fe</mml:mi><mml:mi>M</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>, and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>R</mml:mi><mml:mrow><mml:mi mathvariant="normal">Ga</mml:mi><mml:mi mathvariant="normal">Cu</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>(<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>R</mml:mi><mml:mo>=</mml:mo><mml:mi mathvariant="normal">Yb</mml:mi></mml:mrow></mml:math>and Lu,<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>M</mml:mi><mml:mo>=</mml:mo><mml:mi mathvariant="normal">Co</mml:mi></mml:mrow></mml:math>and Cu)

The magnetic and dielectric properties have been investigated for the isostructural oxides of $R{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$, $R\mathrm{Fe}M{\mathrm{O}}_{4}$, and $R\mathrm{Ga}\mathrm{Cu}{\mathrm{O}}_{4}$ ($R=\mathrm{Yb}$ and Lu, $M=\mathrm{Co}$ and Cu). The magnetization measurements for $R{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ showed ferrimagnetic ordering at $\ensuremath{\sim}250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This system also exhibited large dielectric constants of $\ensuremath{\sim}10\phantom{\rule{0.2em}{0ex}}000--30\phantom{\rule{0.2em}{0ex}}000$ at around room temperature, which is attributable to the charge-ordering-induced ferroelectricity, as was proposed in our recent paper. The magnetic transition temperatures are lowered to $\ensuremath{\sim}45--90\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for $R\mathrm{Fe}M{\mathrm{O}}_{4}$. Magnetic ordering is not found for $R\mathrm{Ga}\mathrm{Cu}{\mathrm{O}}_{4}$. ac magnetic susceptibility measurements indicate that magnetic ordering becomes short-ranged by the substitution at the Fe site. The overall characteristic behavior of the magnetic properties is explained in terms of the change of a spin value as well as the dilution of magnetic interactions. Although the ac dielectric measurements show the existence of polar regions in each material, the dielectric constants below $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ become smaller in the order of $R{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$, $R\mathrm{Fe}\mathrm{Co}{\mathrm{O}}_{4}$, $R\mathrm{Fe}\mathrm{Cu}{\mathrm{O}}_{4}$, and $R\mathrm{Ga}\mathrm{Cu}{\mathrm{O}}_{4}$. From the analysis of the dielectric dispersion, the distribution of the fluctuation time of polar regions is wider in $R\mathrm{Fe}\mathrm{Co}{\mathrm{O}}_{4}$, $R\mathrm{Fe}\mathrm{Cu}{\mathrm{O}}_{4}$, and $R\mathrm{Ga}\mathrm{Cu}{\mathrm{O}}_{4}$ than in $R{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$; a coherent motion of polar regions is suppressed in the substituted systems. By comparison to the results from the magnetic measurements, the dielectric properties are discussed in connection with a charge transfer between the transition-metal $3d$ orbitals, consistently with the proposed mechanism of the ferroelectricity in $R{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$.